So far, coating compositions mainly comprising a melamine resin, such as an alkyd melamine resin or acrylic melamine resin, have been used in coating automobiles, industrial equipment, steel furniture, building interior and exterior, electric appliances, various plastics products and the like.
However, such coating compositions mainly comprising a melamine resin have some problems: a harmful formalin is generated in the step of curing and, since cured coatings are poor in acid resistance, they are affected by acid rain. In particular, the acid rain problem has been aggravated owing to recent air pollution and phenomena such as etching, whitening and staining or spotting of coatings or paint films have been encountered.
To solve the above problems, melamine resin-free coating compositions have already been proposed (e.g. JP Kokai H03-287650 and JP Kokai H02-45577). In such coating compositions, ester bonds resulting from the reaction of acid groups and epoxy groups serve as crosslinking sites and, therefore, said coating compositions are advantageous, as compared with the coating compositions mainly comprising a melamine resin, in that they can form coatings having good acid resistance. However, it has now been found that once the coatings which were formed from said coating compositions by reacting an acid group with an epoxy group is stained, the stains tend to be difficult to remove, although said compositions give coatings having good acid resistance. In particular, the tendency is more remarkable with tint or weak color coatings. Thus, from the viewpoint of good appearance, easy cleaning, reduced number of cleanings, or the like, improvements in stain resistance are desired.
Furthermore, in top coat finishing of automobiles, improvements in finish appearance are earnestly desired.
When it is demanded that coated articles have high quality appearance, it is a usual practice to further incorporate a leveling agent, which is an appearance modifier, in top coat paints. At present, without addition of a leveling agent, none of the commercial paints or coating compositions can give a satisfactory finish appearance or cope with the high quality appearance requirement.
When a leveling agent is added to a composition for coatings capable of providing stain resistance, it is possible that the stain resistance of the coating composition decreases.
WO 98/07797 discloses a curable composition (.alpha.) for coatings which comprises an epoxy group-containing resin (A) component, a carboxyl group-containing compound (B) component and a silicate (C) component as well as a composition derived from said composition by introducing a hydrolyzable silyl group into the (A) component, namely a curable composition (.beta.) for coatings which comprises a hydrolyzable silyl group- and epoxy group-containing resin (A) component, a carboxyl group-containing compound (B) component and a silicate (C) component. However, though such compositions indeed provide good stain resistance, the stain resistance may be lowered by certain additives added in the step of paint or coating preparation. Also in cases where a leveling agent is added as the additive, the same problem may be encountered and it is necessary to select a leveling agent which will not lessen the stain resistance. Alternatively, a paint or coating is earnestly desired which is capable of providing a good appearance without adding any leveling agent, hence capable of retaining the stain resistance without any reduction in stain resistance due to the addition of a leveling agent.